Patent Documents 1 and 2 listed below disclose hollow poppet valves comprising a valve head integrally formed at one end of a valve stem, the poppet valve formed with an internal cavity that extends from within a valve head into the stem and is charged, together with an inert gas, with a coolant that has a higher heat conductivity than the valve material. An example of such coolant is metallic sodium having a melting point of about 98° C.
Since this type of internal cavity extends from within the valve head into the stem and contains a large amount of coolant, it can advantageously enhance the heat conduction ability (hereinafter referred to as heat reduction capability) of the valve.
It is known that if the temperature of a combustion chamber of an engine is heated to an excessively high temperature during an operation, knocking may take place, which lowers the fuel efficiency and outputs, and hence the performance, of the engine. In order to lower the temperature of the combustion chamber, there has been proposed different types of hollow poppet valves which have an internal cavity loaded with a coolant together with an inert gas so as to positively conduct heat from the combustion chamber via such valve (i.e. a method of enhancing heat reduction effect of the valve to remove heat from the combustion chamber by enhanced heat reduction effect of the poppet valves).